Clean up some mess in row-security patches.

Fix unsafe coding around PG_TRY in RelationBuildRowSecurity: can't change
a variable inside PG_TRY and then use it in PG_CATCH without marking it
"volatile".  In this case though it seems saner to avoid that by doing
a single assignment before entering the TRY block.

I started out just intending to fix that, but the more I looked at the
row-security code the more distressed I got.  This patch also fixes
incorrect construction of the RowSecurityPolicy cache entries (there was
not sufficient care taken to copy pass-by-ref data into the cache memory
context) and a whole bunch of sloppiness around the definition and use of
pg_policy.polcmd.  You can't use nulls in that column because initdb will
mark it NOT NULL --- and I see no particular reason why a null entry would
be a good idea anyway, so changing initdb's behavior is not the right
answer.  The internal value of '\0' wouldn't be suitable in a "char" column
either, so after a bit of thought I settled on using '*' to represent ALL.
Chasing those changes down also revealed that somebody wasn't paying
attention to what the underlying values of ACL_UPDATE_CHR etc really were,
and there was a great deal of lackadaiscalness in the catalogs.sgml
documentation for pg_policy and pg_policies too.

This doesn't pretend to be a complete code review for the row-security
stuff, it just fixes the things that were in my face while dealing with
the bugs in RelationBuildRowSecurity.

src/include/rewrite/rowsecurity.h

@@ -21,11 +21,11 @@ typedef struct RowSecurityPolicy
 {
 	Oid					policy_id;		/* OID of the policy */
 	char			   *policy_name;	/* Name of the policy */
-	char				cmd;			/* Type of command policy is for */
+	char				polcmd;			/* Type of command policy is for */
 	ArrayType		   *roles;			/* Array of roles policy is for */
 	Expr			   *qual;			/* Expression to filter rows */
 	Expr			   *with_check_qual; /* Expression to limit rows allowed */
-	bool				hassublinks;	/* If expression has sublinks */
+	bool				hassublinks;	/* If either expression has sublinks */
 } RowSecurityPolicy;
 
 typedef struct RowSecurityDesc